The invention relates to a topology-preserving transposition method for transposing a Region of Interest (ROI) from a source image to a destination image.
The field of the invention is the processing and analysis of medical images.
Regions of Interest, or ROI, are used to isolate or extract areas of interest in medical images, in order to perform measurements or any other tasks on these areas.
Medical images include for instance images issued from Computed Tomography (CT), Magnetic Resonance Imaging (MRI), Positron Emission Tomography (PET), or medical ultrasonography.
Medical images are usually represented as collections of picture elements, such as pixels for the bi-dimensional (2D) images or voxels for the tree-dimensional (3D) or volume images.
ROI can be represented as a collections of picture elements identified or labelled as belonging to, for instance, an anatomical structure (lung, blood vessel, . . . ) or a pathological structure (nodules, . . . ). ROI can be created manually, by an operator who identifies the picture elements corresponding to a specific structure and tags them. They can also be created by means of automated or semi automated methods using picture analysis tools and a-priori knowledge.
During clinical exams, it is often desirable to transfer a ROI from one image or one series of images to another, for instance for a longitudinal follow-up of a patient, or for comparison or cross measurement purposes.
The images may be acquired with the same modality or imaging technique at different times, or using different acquisition channel.
They may also be acquired with different modalities which allow for instance accessing to different types of information. For instance, a CT technique may be used to obtain images with structural or anatomical information, and PET technique may be used to obtain functional information on the same anatomical structures.
In general, it is not desirable, or even not possible to re-compute a ROI independently in each of the image series: It would be too time consuming, and there would be a risk that the segmentation process does not lead to a ROI corresponding to exactly the same anatomical structure. Furthermore, sometimes a ROI may be computed in an image series (for instance a structural one) but the anatomical structure may not be recognizable in another image series (for instance a functional one) or distinct enough for segmentation and so the ROI may not be computed in the latter image series.
A solution is to transpose a ROI computed in one image series to another image series. But several problems may appear: the spatial sampling of the images may be different, and there may be deformations of the imaged anatomical structures between the image series. The deformations may be of the rigid type (translations, rotations, scaling) and/or of the soft type (deformations of the anatomical structures into the images). They may be due for instance to differences in image modalities, imaging positions, and/or motions of the patient or of the anatomical structures themselves (breathing . . .).
Soft or rigid deformations between images are usually taken into account during radiological exams through a registration process. Transfer functions corresponding to soft and/or rigid deformations between images are computed, on the basis of manual and/or automated identifications of corresponding structures in the images. But in particular in the case of soft registration (i.e. taking into account soft deformations), these transfer function do not result in the general case to a pixel-to-pixel correspondence between images.
We know for instance the document U.S. Pat. No. 7,817,835 which describes a method for doing cross reference measurements between images. Measurements done in a source image and represented or materialized by mark locations such as segments, circles . . . drawn in that source image, are transposed to a destination image using registration information. Then the same measurements are done independently in the destination image and compared to the transposed measurements.
However, the method is not applicable to ROI transposition because in the general case, a point-by-point transposition of an area in a source image does not lead to a coherent representation of a corresponding area in a destination image. And in addition, the method of U.S. Pat. No. 7,817,835 is limited to rigid registration.
It is an object of the invention to provide a method for transposing a ROI from a source image to a destination image using soft and/or rigid image registration information.
It is a further object of the invention to provide a method for transposing a ROI from a source image to a destination image which allows optimizing the calculation requirements.
It is a further object of the invention to provide a method for defining a ROI in a destination image in which it cannot be easily obtained, using a ROI obtained in a source image.
It is a further object of the invention to provide a method for defining a ROI in a destination image using a segmented reference image.